Spectral domain optical coherence tomography (SDOCT), including both spectrometer-based and swept-source systems, has demonstrated clinical potential for in vivo high-resolution and high-speed imaging of biological structures. Advances in Doppler SDOCT have demonstrated several image acquisition schemes that enabled real-time, high-resolution, volumetric display of blood flow maps. These techniques, while able to provide 3D flow maps and velocimetry data, are inherently oversampled and therefore have reduced imaging speed and are more susceptible to sample motion.
Current generation Doppler SDOCT techniques use phase differences between sequential A-scans acquired at a single lateral position to calculate the velocity of moving scatterers through depth. These techniques relate the phase differences between sequential interferograms to a Doppler frequency-shift which, in turn, is related to the velocity of the moving scatterers. Recently, advances in Doppler SDOCT have led to a joint spectral and time domain acquisition scheme (STdOCT), which allows for near phase-noise limited velocity resolution in low signal conditions. This technique is a variation of conventional Doppler, which functions by determining the velocity of moving scatterers using their temporal frequency shifts rather than the phase differences between sequential A-scans at a single lateral scan position. By taking a 2D Fourier transform of interference fringes temporally oversampled at the same A-scan position 20-40 times, STdOCT directly maps wave number to depth and time to Doppler frequency, thus creating a depth-resolved Doppler velocity map. This technique, while able to provide depth-resolved flow images and velocimetry data, is several times more oversampled than conventional Doppler, making it more susceptible to motion artifacts. Other techniques for identifying vessels involve injections of contrast agents such as FA and ICGA, and/or manual segmentation of vessels.
Spatial frequency modulations across lateral scans have been introduced as a method for full range complex conjugate resolved imaging. Similar to previously described complex conjugate resolving techniques using electro-optic phase modulators and acousto-optic frequency shifters, the spatial frequency modulation technique separates real and complex conjugate reflectivities by imposing a spatial carrier frequency laterally across a B-scan. The carrier frequency is generated by adding a phase delay to each A-scan using a moving reference arm or an off-pivot scanning beam. Similarly, a 3D optical angiography technique has been demonstrated by using a modulated reference arm delay and by detecting scatterers not modulated at the carrier frequency as a result of flow-induced Doppler frequency-shifts. Resonant Doppler flow imaging also uses reference arm modulations to detect flow, but instead of using a moving reference arm mirror, resonant. Doppler uses an electro-optic modulator, driven at a flow detection frequency, to phase-match the reference signal to that of the moving scatterer.
Spatial frequency flow detection techniques can be considered optical analogs to power Doppler (PD) ultrasonography. Developed as a method of improving the sensitivity of Doppler ultrasound, the analog to DOCT, PD reports the power of the Doppler signal within specified frequency windows instead of the mean frequency shift. The advantage arises from the representation of the power spectrum of random phase noise. Since the noise in the power spectrum is uniformly low, random phase variations can be filtered out by raising the sensitivity threshold above the noise floor. The Doppler signal in PD is represented as an integral of the power spectrum, which improves the sensitivity and detection range of moving scatterers at the expense of eliminating velocity information. PD is relatively insensitive to Doppler angle and phase wrapping since these factors only modify the distribution of the Doppler power spectrum, but the total integrated power remains constant. The Doppler signal in PD is separated from non-moving components by filtering out all power spectrum components without Doppler shifts, thus only imaging moving scatterers. The resulting PD signal is related to the number of moving scatterers producing the corresponding Doppler shifts.
These techniques rely on precise synchronization of reference arm modulation and B-scan acquisition, require expensive or cumbersome modulators, and are unable to detect bidirectional flow in a single B-scan pass. In addition, spatial frequency filtering techniques, while able to provide 3D flow maps with improved acquisition speed and sensitivity compared with DOCT, lack velocity-resolved blood flow information provided by techniques such as DOCT and laser Doppler velocimetry (LDV). Accordingly, in ophthalmology, noninvasive quantification of blood circulation in tissues would be advantageous to facilitate the description of retinal vascular changes prior to and during ocular and systemic disease.